1. Field of the Invention
The present invention relates to a detector for detecting information on displacement of a moving object or fluid, or a velocimeter for measuring the velocity of a moving object without contacting them. The present invention relates in particular to a Doppler velocimeter for detecting velocity by detecting the frequency shift of light.
2. Related Background Art
Traditionally, the laser Doppler velocimeter has been used to perform a noncontact high-precision measurement of the traveling velocity of a moving object or fluid. The laser Doppler velocimeter for measures the traveling velocity of the moving object or fluid by utilizing the effect (Doppler effect) that the frequency of rays of light scattered by the moving object or fluid is shifted in proportion to its traveling velocity when a laser beam is irradiated onto the moving object or fluid.
FIG. 1 is a view illustrating an example of the conventional laser Doppler velocimeter.
In FIG. 1, the laser light emitted from the laser light source 1 is made into the parallel beams 3 by the collimator lens 2, and split into two beams 5a and 5b by the beam splitter 4. The split beams 5a, 5b are reflected by the reflection mirrors 6a and 6b respectively and are irradiated at an incident angle .theta. by the beam irradiation onto one and a same position on a moving object or fluid 7 at a traveling velocity V. The scattering lights from the moving object at that time are detected by the photodetector 9 through the condenser lens 8. The frequency of the scattering rays of light produced by the two beams is proportional to the traveling velocity V and is affected by the Doppler shift of +.DELTA.f and -.DELTA.f, respectively. Now, given the wavelength of the laser light as .lambda., the .DELTA.f can be expressed in an equation (1) given below. EQU .DELTA.f=V sin (.theta.)/.lambda. (1)
The scattering rays of light affected by the Doppler shifts +.DELTA.f and -.DELTA.f, interfere with each other to result in the variation of light intensity on the light receiving plane of the photodetector 9. Its frequency F is obtainable by an equation (2) given below. EQU F=2.DELTA.f=2V sin (.theta.)/.lambda. (2)
With the measurement by the equation (2) of the frequency F (hereinafter referred to as Doppler frequency) of the photodetector 9, the traveling velocity V of the moving object 7 can be obtained.
In such a laser Doppler velocimeter as in the above example of the conventional art, the Doppler frequency F is inversely proportional to the laser wavelength .lambda. as seen from the equation (2). Therefore, it is necessary for the laser Doppler velocimeter to use a laser light source capable of emitting a light having stable wavelength. As a laser light source capable of continuous transmission with a stable wavelength, a gas laser such as He - Ne is often employed, but the size of its laser oscillator is large and a high voltage power source is needed. Accordingly, the apparatus becomes large and expensive. On the other hand, the laser diode (or semiconductor laser) employed for a compact disc, video disc, optical fiber communications or the like is very small and easy to drive. However, there is a problem of temperature dependence.
FIG. 2 (quoted from Optical Semiconductor Devices compiled in Mitsubishi Semiconductor Data Book 1987) illustrates an example of the typical temperature dependence of a laser diode. The the continuous wavelength changes continuously is caused mainly by the temperature changes of the refractive index of the active layer of the laser diode, which is 0.05-0.06 nm/.degree.C. The discontinuous wavelength changes is called longitudinal mode hopping, which is 0.2-0.3 nm/.degree.C.
In order to stabilize the wavelength, a controlling method is usually adopted to maintain the laser diode at a constant temperature. Such method requires the installation of the temperature control members such as a heater and a radiator; and temperature sensor with small thermal resistance to control the temperature precisely. This makes the laser Doppler velocimeter comparatively large in its size and high in its cost in addition, it is still impossible to eliminate completely the instability resulting from the longitudinal mode hopping.
With a view to solving the above-mentioned problems, a method for detecting the scattering lights from a moving object or fluid by the use of a photodetector has been proposed in U.S. patent application Ser. No. 501499, as a laser Doppler velocimeter, in which the laser light as the light source is incident on a diffraction grating, and the two diffracted lights obtained therefrom, which are +n order and -n order (n is 1, 2, and, ...) except zero order, are irradiated onto the moving object at an intersecting angle which is the same as the angle formed by the two beams.
FIG. 3 illustrates an example of diffraction wherein a laser light I is incident on a transmission type diffraction grating 10 having a grating pitch d, perpendicularly to the direction in which the grating is arrayed, and the diffraction angle .theta..sub.n is expressed by an equation given below. EQU sin .theta..sub.n =m .lambda./d
where m is the refraction order (0, 1, 2, and ...) and .lambda. is the wavelength of light.
Here, the light of .+-.n order other than zero order is expressed by an equation given below. EQU sin .theta..sub.n =.+-.n .lambda./d (3)
where n is 1, 2, and ...
FIG. 4 is a view illustrating the two-beam irradiation of said .+-.n order light onto the measuring object 7 by the use of the mirrors 6a and 6b in such a manner that its incident angle becomes .theta..sub.n. The Doppler frequency F of the photodetector 9 is expressed by an equation given below, obtained from the equations (2) and (3): EQU F-2Vsin .theta..sub.n /.lambda.=2nV/d (4)
The Doppler frequency F does not depend on the wavelength of the laser light I and is inversely proportional to the grating pitch d of the refraction grating 10 and is proportional to the traveling velocity V of the measuring object 7. Because the grating pitch d is sufficiently stable, a frequency proportional to only the traveling velocity V of the measuring object 7 is obtained as the Doppler frequency F. In this respect, when a reflection type diffraction grating is used as the grating 10, same result will be obtained.